1. FIELD OF THE INVENTION
The present invention relates to compositions for inhibiting the polymerization of acrylic acid, and methods of manufacturing the composition. This invention is particularly useful for preventing the polymerization of acrylic acid upon its distillation. Accordingly, the invention also relates to processes for stabilizing the distillation of acrylic acid.
2. DESCRIPTION OF THE PRIOR ART
Although acrylic acid was first prepared in 1847, and although its polymerization has been known for over a century, means for addressing the technical difficulties associated with its manufacture and polymerization have only begun to surface since 1930. These processes for manufacturing acrylic acid, and particularly its concentration and purification, have normally comprised various thermal distillation techniques. It is widely known that the principal losses of acrylic acid in these operations are due to thermal polymerization of the acrylic acid monomers. However, since the thermal energy is essential in these distillation operations, various devices or means for inhibiting the polymerization despite the presence of heat have been variously considered over the years. Notably, mild operating conditions, rapid through put avoiding residence time of the acrylic acid, and feeding polymerization inhibitors into the distillation device have been employed for this purpose.
In the past, a number of polymerization inhibitors have been tried, either singly or as combinations of two or more and often synergized by molecular oxygen bubbled through the distillation column during the course of aerobic distillations. Such inhibitors include para methoxy phenol (herein referred to as hydroquinone-mono-methyl-ether (MEHQ)), phenothiazine (PTZ), catechol, methylene-blue, diphenylamine, various organic copper salts, particular aromatic nitroso compounds and other phenolic, amine, nitro, and quinone compounds and their inorganic salts. However, for the most part, these polymerization inhibitors have been specific for vapor phase polymerization inhibition or alternatively liquid phase polymerization inhibition.
U.S. Pat. No. 3,674,651 discloses a polymerization inhibitor composition for acrylic acid comprising molecular oxygen, diphenylamine or its derivatives and either benzoquinone or MEHQ. However, the process for synergizing the composition calls for introducing the inhibitors at the top vapor phase of the column, while adding an additional general liquid phase polymerization inhibitor to the liquid phase and supplying air or oxygen up through the bottom of the column to effect the stabilization.
U.S. Pat. No. 4,021,310 discloses a polymerization inhibiting procedure for acrylic acid in vapor-phase catalytic oxidation requiring at least one inhibitor such as MEHQ or PTZ combined with particular carbamate or salicylate copper salts and air or molecular oxygen bubbled into the distillation column.
In 1976 Mr. Myron J. Jursich theorized in his U.S. Pat. No. 3,959,358 that phenol-type polymerization inhibiting compositions such as MEHQ (para methoxy phenol) for acrylate esters were made effective in the presence of dissolved oxygen if combined in an excess molar amount with various amine type inhibitors such as PTZ. He theorized that phenol-type inhibitors satisfactorily prevent thermally activated polymerization of acrylic esters when little or no oxygen is present. However, according to Jursich, when oxygen is present in the ester solution such as from polymerization initiating species like peroxides, the phenol-type inhibitor alone is inadequate. Accordingly, various amine-type inhibitors, particularly phenothiazine (PTZ), were used to scavenge the dissolved oxygen without air or oxygen bubbled into the column. However, unlike acrylic esters, this theory has been found to be ineffective with acrylic acid.
N-nitrosophenyl-hydroxylamine (NPH) has been known as a stabilizer inhibiting the polymerization of monomers for a number of years. See for example, U.S. Pat. No. 2,758,131 and German Pat. No. 1,180,733. Also, U.S. Pat. No. 3,426,063 discloses its use in inhibiting the polymerization of water soluble acrylate esters but not acrylic acid. In the past, the use of NPH as a polymerization inhibitor for acrylic acid has had a number of drawbacks including the need for using it in such higher concentration that the water of dissolutions has a deleterious effect on the distillation operation.
Mr. Thomas Stewart discloses in his U.S. Pat. No. 4,210,493 that in certain anaerobic conditions common to acrylic acid distillation such as vapor spaces, overheads, unvented reflux columns, and especially when vacuum distillation is involved, a lack of oxygen flowing through the equipment renders total loss of inhibitory properties to most known inhibitors when attempting to inhibit polymerization of acrylic acid. Certain aliphatic C-Nitroso compounds are said to overcome that problem for acrylic acid vapor phase polymerization inhibition where there is no air or oxygen present.
U.S. Pat. No. 4,310,676 discloses another polymerization inhibitor composition for acrylic acid under anaerobic sealed storage conditions comprising phenothiazine (PTZ) and paranitrosophenol.
A new and improved polymerization inhibiting composition for acrylic acid in both the vapor and liquid phases devoid of the prior art problems would be a substantial advancement in the art.